The present invention relates to a method and system for playback of recorded informaton, and more particularly to the playback of information recorded on a disk during a special playback operation.
Video disks for recording video information are so arranged that recesses (referred to as "pits") or protrusions form concentric recording tracks or a spiral recording track on the disk surface. The recorded information extends along the length and the intervals between such pits or protrusions.
From the video disk having the foregoing construction, the information recorded thereon may be picked up and reproduced, for example, by demodulating a pickup signal produced by a light beam reflected or transmitted from the recording surface of the recording disk, the signal being modulated by the presence or absence of the pits. In the device for picking up information from such type of recording disk, a tracking servo system is provided for driving a tracking actuator so that the information reading point of the pickup device accurately traces the information recording track on the recording disk at all times. Generally, such a reading operation of an information reading system includes normal playback during which frames of the video information, each of which includes two fields of video signals, are read out in order. A special playback operation is involved, such as still picture playback wherein the same frame of the video information is read out repeatedly. In such special playback operations, the slow speed playback phase and the fast speed playback phase are enabled by controlling the jump of the information reading point of the pickup device and a method is utilized to open the tracking servo loop while the tracking actuator is positively driven at the same time. The information reading point is thereby moved from the recording track being followed and subsequently brought to another recording track portion to be traced while the servo loop is reclosed. Further, the information recording disk associated with the foregoing type of system, is either of the CAV (Constant Angular Velocity) type on which information is recorded so that the reading of such information is performed while the disk is rotated at a constant angular velocity, or of the CLV type wherein the disk is rotated in order to obtain a constant linear velocity readout. In the case of CAV type disks, any circumferential section of the recording track at some radial location on the disk carries a constant amount of information, e.g. the video information of one frame. Therefore, portions of the recording track on which the information corresponding to vertical synchronizing signals disposed between two adjacent frames are aligned in radial relation to the rotational axis of the recording disk. Therefore, with the CAV type video disks, the synchronizing signal segment of the video signal is regularly reproduced, even immediately after a track jump operation, which is essential for the special playback operations. As a result, the special playback operations can be performed satisfactorily without any difficulty.
In the case of the CLV type disks, the amount of information recorded during one revolution of the disk, i.e. in one circumferential track portion, varies depending on the radial position of the recording disk. In other words, the ratio between the period of one revolution of the disk and the period of one field of the video signal is not maintained at an integer value. For example, video information of one frame is recorded in the innermost recording track section while video information of three frames is recorded in the outermost recording track section. Therefore, with the CLV type disks, the generation of the synchronizing signal segment is affected in the video signal obtained immediately after a track jump operation. Thus, it is difficult to maintain the continuity of the video signal for satisfactory picture reproduction if the track jump operation, essential for the special playback operation is performed. In order to solve such problem, it has been proposed to record the readout video signal in a video memory, and to use as a playback video signal, the video signal readout from the video memory by controlling its read address so as to maintain the regularity of the synchronizing signal.
However, the foregoing proposed solution is not easily achieved. In systems for playback of information recorded on a recording disk, a CCD (Charged Coupled Device) is provided for controlling a spindle motor which rotates the recording disk and to effect time base correction. The frequency of a clock signal supplied to the CCD is varied in accordance with a phase difference between an output signal of a synchronizing signal separator, separating synchronizing signal from a playback video signal and an output of a reference oscillator. By such control of the frequency of the clock signal, the output signals of the synchronizing signal separator and the reference oscillator are synchronized in phase. However, in the case of CLV type disks, wherein the positions of the synchronizing signal are not radially aligned, a non-constant phase error develops between the output signal of the synchronizing signal separator and the reference oscillator in association with the track jump operation. As a result, a certain time period is required before the phase of the playback video signal establishes a predetermined relationship to the phase of the output signal of the reference oscillator. Moreover, when the information reading point is moved radially by a track jump operation, within a short time, over a CLV type disk, a transient response of the CCD and spindle motor occurs to maintain the linear velocity. Therefore, additional time is required for stabilizing such transient response.
As an example of a technique for decreasing the time base error associated with the track jump operation of a CLV type disk, a method is proposed by the inventors of the present application, described in U.S. Pat. No. 4,609,948. However, even with such a method, a time period varying from several milliseconds to ten milliseconds is required in order to absorb the time base error completely. Therefore, it becomes necessary to provide an additional device for correcting the time base error of the signal read out from the memory in order to eliminate the disturbance of the synchronizing signal during a special playback mode or another device for correcting such disturbance before writing of the video signal into the memory is separately required. Thus, a complexity problem arises in the construction of the information playback system.
An object of the present invention is therefore to provide a method and system for playback of recorded information wherein special playback operations are enabled while maintaining the construction of the system relatively simple.
Another object of the present invention is to provide a method and system for playback of information recorded on a non-CAV type recording disk by using a memory device in which the length of the information to be recorded may be determined without regard to the radial direction of movement of the information reading point on the recording disk.